Anti-clogging apparatus and method for maximal flux and prolonged filtration time

ABSTRACT

An apparatus and method for filtering a suspension including particulates and increasing the concentration of retainable matter wherein the suspension is drawn through the filter in a manner such that a portion of the suspension is subjected to a negative pressure which pulls it away from the filter membrane and permits it to pass through the filter unit without going through the membrane and a portion of the suspension is subjected to a positive pressure which causes it to pass through the filter membrane before exiting the filter unit, thereby creating a permeate. The invention involves regulation of flow rates to achieve a ratio of the pressures so that when some of the particulate matter from the portion of the suspension passing through the filter membrane becomes effective in decreasing flux, the negative pressure will serve to pull the particulate free and unclog the filter membrane on a continuous and automatic basis, thus allowing prolong filtration time with maximal flux.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to filtration systems. Filtration systemshave conventionally been used for either (1) removal of particulatematters from fluid suspensions to result in clear, non-turbid fluids or,(2) removal and discarding of part of the soluble and fluid fraction forthe purpose of concentrating the desirable particulate matters. Toachieve the first purpose, the filter in the system is used to trap theparticulate matters by virtue of the effective pore sizes being smallerthan the particulate matters, while allowing the soluble fraction to gothrough the filter pores and collected for subsequent use. To achievethe second purpose, the ideal filter will allow the soluble fraction togo through the filter pores with only minimal entrapment of theparticulate matters which are then returned to collection containers asthe "retentate" fraction for subsequent use. In both procedures,clogging of the filter remains a major problem.

2. Description of the Prior Art

The clogging of filter pores is a major problem with prior artfiltration and dialysis devices. Clogging of the filter pores quicklyreduces the efficiency of the filtration system. As the number ofunclogged pores diminishes, filtration rate decreases. Since flow rateis equal to pressure gradient divided by resistance, as more and morefilter pores are clogged (increasing resistance), a progressively largerpressure gradient is needed to maintain adequate flow rates. Even then,when enough of the filter pores become clogged, flow rate will becomefor all practical purposes, zero. At that point, particulate matters canno longer be removed from fluid suspensions. In addition, if the purposeis to concentrate suspended particulate matters, clogging of filterswill decrease the final yield of the particulate matters and may in factdecrease the concentration of such matters in the retentate.

To minimize the problem of clogging, various approaches have beendesigned, as reflected in different filtration systems on the market.One approach incorporates designs for vigorous stirring of thesuspension physically above, or prior to interaction of the suspensionwith the filter surface. Examples include the Stirred Cells Series ofAmicon Division, W.R. Grace & Co. However, such systems are ineffectivebecause the distance between the stirring mechanism and the filtermembrane (typically in millimeters) are several orders of magnitudelarger than the diameter of the particles (typically in microns). Oncethe particulate matters are trapped within the filter pores, withconstant positive filtration pressure pressing them against the filtermembrane, agitation at a far distance (relative to the size of theparticulate matters) will not effectively dislodge them. Moreover, highshearing forces generated by vigorous stirring may cause foaming anddenaturation of the particulate matters.

Another approach involves the concept of tangential flow as exemplifiedby Millipore's Minitan system. Instead of applying pressureperpendicular to the surface of the filter, the suspension is pushedforward by positive pressure from a pump system so that it travels in adirection tangential to the filter surface. In theory, this designallows the particulate matters to travel in a direction tangential tothe filter surface while the soluble phase goes through the filter poresin a direction perpendicular to the filter surface. In practice,however, substantial clogging still occurs. The reason is that theparticulate matters are carried by the soluble phase of the suspensionand will travel in the direction of the immediate fluid surrounding agiven particle. Any time a fraction of the soluble phase goes throughthe filter pores (in a direction perpendicular to the filter surface), aproportional amount of particulate matters will travel with it in thesame direction. Regardless of the direction of flow of the rest of thesuspension bulk (which may travel in a direction tangential to thefilter surface), the fraction that goes through the pores will clog upthe pores. With this understanding, it becomes clear that tangentialflow filter systems are only different ways of recirculating the bulk ofthe suspension before its interaction with the filter pores. This designdoes not substantially alter the clogging potentials of particulatematters at the level of the filter pores because the particulate mattersare again pressed onto the pores by the positive pressure used tocirculate the bulk of the suspension.

Since both the stirred cell design and the tangential flow systems usepositive pressure to circulate the suspension, they both result intrapping of particulate matters within the matrix of the filtermembrane. For this reason, these systems are not suitable for thepurpose of concentrating particular matters. There exists a need for anovel design where: (1) the filter membrane will not be clogged, and (2)should unexpected change in the filtration condition lead to someclogging, the obstructed pores will become unclogged again. Such adevice will allow efficient concentration of valuable particulatematters. In addition, because of the increased life span of the filtermembrane, it also allows cost-efficient collection of the soluble phaseof the suspension, if the soluble phase is the desirable fraction fromthe suspension.

In special filtration systems such as ultrafiltration for handlingultrafine particles suspended in a fluid phase, the filter membrane isoften anisotropic, consisting of a thin skin supported by a porousbacking. As such, the skin is the barrier which separates the retentatefrom the permeate and does not necessarily have microscopically visiblepores. However, positive pressure forcing the fluid out from theretentate through the skin to become the permeate promotes theaccumulation of particles next to the skin which impedes furtherimprovement in filtration efficiency. This is known as the gelresistance. The greater the positive pressure to squeeze the fluid outof the skin, the greater is the gel resistance. When the filtrationefficiency (defined as "flux" which is often in units of gallons ofpermeate obtained per square foot filter area per day, or GSFD) isplotted against transmembrane pressure (pressure difference between theretentate surface and the permeate surface of the filter), initiallyflux increases with increase in transmembrane pressure. Rententatesurface is defined as that surface of the filter which faces theretentate or particulate suspension. The permeate surface is the othersurface of the filter which faces the permeate or filtrate fluid.Particles are defined here as materials carried by the fluid phase to beseparated by the filter membrane. Particles can therefore be in adissolved state, or colloidal or solid state. However, after a certaintransmembrance pressure is obtained, flux increases no more and becomesa constant value. This is due to the increase in resistance which buildsup as transmembrane pressure is increased. Very often, due to the largeparticle sizes as compared to the sub-microscopic pore sizes of theskin, the filter is not described as being clogged, but is describedtechnically as having resistance (e.g. gel resistance) that impedesfurther increase in flux. This technicality is recognized and will beincluded in this application as part of the "clogging" process sinceclogging is defined here as any process that decreases the filtrationefficiency of the filtration system.

To overcome the problem of clogging, examples in the prior art includethe following:

1. Pre-filters are used to pre-sieve large particles and to decrease thetotal load presented to the main filter. Pre-filters have, by necessity,pore sizes larger than the pore sizes of the main filters. This methoddoes not prevent the clogging of either the main filter or theprefilter. This method is actually two filtration systems working intandem or in sequence with all the classical problems associated withsuch systems. Once some pores become clogged, the number of poresavailable for filtration decreases and the particles are often trappedinside the filter matrix and not recirculated back to the retentate.

2. Tangential flow is an attempt to decrease the clogging of filterpores by introduction of a sweeping force tangential to the surface ofthe membrane instead of perpendicular to it to sweep away any cloggingmaterial. This is performed by pushing the fluid in a directiontangential to the membrane surface instead of directly onto it(perpendicularly to it). However, such systems inevitably use positivepressure to squeeze the fluid out of the filter. Positive pressure isdefined here as pressure within the filtration system where particlesare pushed onto the retentate surface of the filter. Positive pressurecan be created by either a pump upstream from the filter system forcingthe suspension onto the retentate surface to "squeeze" the fluid out aspermeate, or by a vacuum system downstream from the permeate surface ofthe filter "sucking" the permeate out. To the extent that the fluidentering the pores is travelling in a direction perpendicular to thesurface of the filter, the particles carried by that portion of thefluid will also travel in a direction perpendicular to the surface ofthe filter and therefore will clog the pores. The remaining portion ofsuspension (in fact the bulk of suspension) that does not interact withthe pores of the filter will not clog the pores, and therefore it doesnot matter what direction it travels as far as clogging is concerned.Therefore, tangential flow is only a method of moving the bulk ofsuspension which has no immediate effect on clogging and does not solvethe problem of clogging at the level of the pore sites.

3. Back flushing--many filtration systems are obligated to terminate thefiltration process when the filtration efficiency drops below acceptablelevels. Instead of discarding the expensive filters, a fresh fluidmedium is forced from the permeate surface of the filter toward theretentate surface to back-flush out the material that clog the pores.This method has several disadvantages: (a) back-flushing time meansdown-time for the filtration process; (b) back flushing is inefficient.Let us consider two clogged pores; pore X is 99% clogged and pore Y isonly 10% clogged. At a given back-flushing pressure, pore Y is going tobe unclogged first. After pore Y becomes unclogged, the back-flushingfluid can go through pore Y with little resistance. Since fluids tend totravel in the pathway of least resistance, there is now even lesspressure to unclog pore X once pore Y has been unclogged. Therefore, thepores that need most unclogging get the least unclogging pressure; (c)Since the particles are trapped during the filtration process, they donot recirculate back to the retentate. If the purpose of the filtrationis to increase the concentration of the particles, trapping of theparticles within the matrix of the filter defeats that purpose; (d) Someparticles are pressure-sensitive. Continued impaction within the poresof the filter may irreversibly damage the particles.

The present inventor, Dr. Richard C. K. Yen has filed two presentlyco-pending patent applications concerning filtration systems. They areas follows:

1. U.S. patent application Ser. No. 07/292,991 filed 01/03/89 entitled"Anti-Clogging And Dialysis Device For Filtration Systems".

2. U.S. patent application Ser. No. 07/311,345 filed 02/16/89 entitled"Vacuum Suction Type Anti-Clogging And Dialysis Device For FiltrationSystems".

The anti-clogging devices described in the co-pending applicationsutilize a negative pressure that constantly unclogs any pores that areabout to clog while filtration goes on. Negative pressure is definedhere as any pressure in the filtration system that promotes movement ofparticles away from the retention surface of the filter. This can beachieved by either a pump downstream from the retentate surface of thefilter "pulling" the particles away from the retentate surface of thefilter, or a pump pushing fluid from the permeate surface of the filtertoward the retentate surface.

These prior art applications have several distinguishing features: (a)the anti-clogging process is an on-going continuous process. It is not atwo step approach of conventional filtration to be followed byback-flush; (b) the anti-clogging process is efficient. It has thegreatest anti-clogging potential where clogging is most likely to occurin the filtration system; (c) anti-clogging method can be used onparticles that are pressure-sensitive because the pores do not trap theparticles; (d) all the pores are continuous being cleansed and thereforethe filter has the maximal number of functional pores and can achievethe maximal filtration efficiency at all times.

It was stated in the pending applications that volume-regulated pumpsare preferred. This is to ensure that whatever pressure that the pumpgenerates to either recirculate the retentate or to extract thepermeate, some constant flow of either permeate or retentate can bemeasured and has values greater than zero. A discussion of the ratio ofrecirculation flow rate versus permeate (filtrate) extraction raterecognized that for different suspensions, this ratio may vary. It wasemphasized that the flow rate of the retentate must be greater than theflow rate of permeate extracted from the suspension in order to generatea negative pressure always greater than the positive pressure pushingthe particles against the retentate surface of the filter. The presentinvention involves an improvement on this process to modify theprinciples disclosed and claimed in the copending applications for usewith other types of suspensions.

SUMMARY OF THE PRESENT INVENTION

To minimize the problem of clogging and to achieve undiminishingfiltration rates and to concentrate the desirable retainable material,the present invention anti-clogging device has the following essentialfeatures:

1. Instead of using positive pressure to push the suspension onto theretentate surface of the filter (whether the bulk of suspension travelsin a direction perpendicular to or tangential to the surface of thefilter), this device employs a negative pressure to pull the suspensionaway from the retentate surface of the filter membrane. The definitionof positive pressure is hereby defined as pressure forcing thesuspension toward the retentate surface of the filter. The retentatesurface is that surface of the filter which faces the suspension andretentate, and which faces away from the filtrate or permeate. By thisdefinition, conventional filtration systems using a negative suctionfrom a vacuum source downstream from the filter unit in fact alsoapplies a positive pressure on the particulate matters onto the filterretentate surface. The present invention, by contrast, is trulyanti-clogging and is novel because it is designed to actually pullparticulate matters away from the retentate surface of the filters.

2. The forces of negative pressure applied on the particulate matterspulling them away from the retentate surface of the filter will be thegreatest in locations within the filtration system where suchparticulate matters are most likely to clog the filter.

3. While most of the filtration units on the market depend on positivepressure generated by pump systems to deliver the suspensions to thefilter unit, some smaller filter units use a vacuum source to pull thesuspension onto the retention surface which is still a positive pressureonto the filter membrane to thereby cause clogging. One example of sucha vacuum--operated filter unit is the Sterifil-D Filter Units(Millipore) comprising a two-compartment structure, with the topcompartment serving as a holding reservoir, separated by a filtermembrane from the bottom (filtrate-collection) compartment. The vacuuminitially generated by the vacuum source via an outlet on the bottomcompartment creates a positive pressure on the retentate surface of thefilter membrane through which the filtrate is drawn from the suspensionin the top compartment.

In contrast, the present invention incorporates a novel design to beused in conjunction with a vacuum source to thereby create a source ofnegative pressure on the retentate surface of the filter membrane, whichwill pull the particulate matter away from the filter retentate surface.We define here as particulate matter any particles that are too large topass through the filter pores and have the potential of clogging thefilter membrane. Therefore, by this definition, particles can bemacroscopically visible solids, or microscopically invisible dissolvedmolecules (e.g. protein macro molecules) which can clog up speciallydesigned molecular-sieving filter membranes.

It has been discovered, according to the present invention, that thecreation of a source of negative pressure in a direction opposite to thedirection of flow of the filtrate creates a force to pull particulatematter away from the filter membrane rather than push it into the filtermembrane, thereby providing an anti-clogging device which assures thatthe filter will have a long and useful life.

It has additionally been discovered, according to the present invention,that for suspensions that are viscous or have particles that clog thepores easily or are pressure sensitive, the flow of retentate shouldalways be greater than the flow of permeate extracted from thesuspension in order to generate a negative pressure sufficiently greatto unclog the system while at the same time allowing filtration tocontinue.

It has further been discovered, according to the present invention, thatthe most efficient filtration system is the one with the maximal fluxand which can maintain that flux for the longest time. In technicalterms, when the flux (GSFD) is plotted against longevity of the process,the system with the largest "area under the curve" is the bestfiltration system. Longevity of filtration can be expressed in a varietyof ways: time of filtration, absolute amount of permeate extracted,volume of permeate extracted as a percentage of initial suspensionvolume, or increase in the percent solid left in the retentate. In otherwords, a filtration system with a very high flux (e.g. by using veryhigh transmembrance pressure) which diminishes quickly due to cloggingwill be less efficient overall than another filtration system where theinitial flux may be only half the value of the first filtration systembut can maintain that flux value for more than twice the time of thefirst filtration system. It is therefore an objective of this presentinvention to achieve the best "area under the curve" using the presentinventor's anti-clogging method.

The present inventor's previous co-pending patent applications primarilyconcerned how to prevent the particles in suspension from clogging thepores. The methodologies disclosed described how to maximally avoidclogging by the particles in the suspension. In contrast, this inventionconcerns the fluid phase of the suspension, on how to obtain thecombined maximum from the maximal flux with the longest time possiblewith that flux value, to achieve the best filtration efficiencyconsistent with an anti-clogging condition.

It has been discovered that several mechanisms contribute to a lowerflux than optimal for a filtration system. (1) Inadequate transmembranepressure. When a new filter is used, all of the pores are open and soclogging is not yet a problem. Flux is determined by the amount oftransmembrane pressure. Since the permeate is flowing from the retentatesurface towards the permeate surface, by definition, the transmembranepressure must be positive pressure. If the transmembrane pressure isnegative, the fluid will flow from the permeate surface toward theretentate surface. Under such conditions of positive pressure, thehigher the transmembrane pressure the larger the flux. If an inadequatetransmembrane pressure is used, less than maximal flux will result. (2)Soon after the start of filtration, clogging becomes a problem. Asdefined above, the higher the transmembrane pressure, the fasterresistance builds up (including clogging and gel resistance). After acertain point, resistance in the system will oppose further increase influx. This equilibrium point presents the maximal flux obtainable with agiven type of filter filtering a given type of suspension. Soon afterthe start of the filtration, some pores will become unavailable forfiltration due to clogging, resulting in a decrease in flux. Additionaltransmembrance pressure will not be effective because it is a problem ofdecreased effective filtration area of the filter.

In the present inventor's anti-clogging method, the problem present initem (2) directly above may not be a problem since Dr. Yen's method isdesigned to maximally prevent clogging. However, it has been discoveredthat a different mechanism can contribute to a lower flux than optimal.When the flow rate of the retentate is set at a value higher thanoptimal, the pump downstream from the filter unit can "pull" thesuspension with enough force to overcome the other pump extracting thepermeate from the suspension. Dr. Yen's previous disclosure set thecondition that permeate extraction rate is always above zero (filtrationprocess going on) while still maximally preventing clogging. It is anobjective of the present invention to disclose the importance of anoptimal retentate recirculation rate compared to permeate extractionrate to obtain optimal flux value. Too high a recirculation rate willgenerate a negative pressure too competitive for the fluid phase (inaddition to the anti-clogging effect on the particles suspended in thefluid phase) resulting in preferential exit of the fluid at theretentate port as compared to exit at the permeate port and thereforeresults in lower flux than optimal.

It has further been discovered, according to the present invention, thatfor suspensions that have low solid contents, or have particles thatwill clog the filter but do so only lightly and easily retrievable fromthe pores, or particles that are hard or pressure-resistant, the flowrate of permeate (filtrate) can exceed the flow rate of the retentate(or retentate recirculation rate). The optimal ratio of retentaterecirculation rate versus permeate retraction rate depends on variousfactors including the nature of the filter material (sticky or not), theparticle sizes, the pore sizes of the filter, the initial percentage ofsolid material in the suspension, the maximum percentage of solid thatcan be tolerated before the retentate becomes too thick forrecirculation, etc.

It has additionally been discovered, according to the present invention,that when the permeate/retentate flow rate ratio is higher than optimal,clogging will be a problem leading to a decrease in flux. This can beverified by the fact that an increase in the permeate/retentate flowrate ratio will further decrease the flux but that a decrease in thatratio will increase the flux.

It has additionally been discovered, according to the present invention,that when the permeate/retentate flow rate ratio is lower than optimal,competition of fluid by the retentate recirculation pump will be aproblem also leading to a decrease in flux. This can be verified by thefact that an increase in the permeate/retentate flow rate ratio willimprove the flux but that a decrease in that ratio will decrease theflux.

Therefore, it is recommended that the following steps be followed: (1)After the filter unit has been primed with an appropriate fluid, turn onthe recirculation pump (flow rate regulated) at the lowest flow ratethat can start the retentate recirculation process. If the pump is notflow rate regulated but pressure regulated, use the lower pressuresufficient to recirculate the suspension. (2) Set the permeateextraction pump at a flow rate setting (if flow rate regulated) orpressure setting (if pressure regulated) which will generate anacceptable flux; start the filtration by turning on the permeate pump.(3) Then turn up the recirculation pump to gradually increase therecirculation rate until a decrease in flux is observed. If step 2 hasalready led to some kind of clogging within a short time, increasing therecirculation rate of the retentate will improve the flux. In that case,continue to step up that recirculating rate until the maximal flux isreached. (4) Maintain the optimal ratio of permeate/retentate flow rateuntil such a time the flux drops again. Then repeat steps 2 and 3 withlarger flow rates or pressure settings. The reasons for a differentoptimal ratio are varied but mostly related to the increased percentageof solids that are now concentrated in the retentate. With increasedpotential of clogging, the retentate flow rate must be increased todecrease that problem and a commensurate permeate extraction rate mustbe arrived at to obtain maximal flux value again.

It has also been discovered, according to the present invention, thatfor a given suspension containing a certain percentage of solid, themaximal flux may be obtained by a permeate extraction rate setting of Pgallons per unit time with the permeate extraction pump and a retentaterecirculation rate setting of R gallons per unit time with the retentaterecirculation pump. If these pumps are pressure regulated and notflow-rate regulated, whatever pressure setting with the permeateextraction pump to produce a whatever pressure setting with theretentate recirculation pump to produce a retentatcate recirculationrate of R gallons per unit time will produce similar results. The sameratio of respective flow rate settings can be maintained also byproportionally increasing both rate settings, e.g. by usingapproximately 2P permeate extraction rate setting and simultaneously aretentate recirculation rate setting of approximately 2R. The advantageof using 2P/2R instead of P/R is that the former rate settings will havea greater anti-clogging potential even though the flux of the system issimilar to using P/R. This is because the tangential vector that carriedthe unclogged particles away from the previously clogged or partiallyclogged pore has now been increased. This arrangement differs from theclassical tangential flow in that the classical tangential flow usespositive pressure which provides gel resistance formation. The presentinvention system incorporates a tangential vector but it uses negativepressures which promotes dispersal of any gel resistance. The tangentialforce merely sweeps any loose material away from the surface of thefilter membrane. The limitation to unnecessarily high flow ratesincludes the wear and tear of the pumps at high rates and electrical andmaintenance costs.

It is therefore an object of the present invention to provide anapparatus and method whereby a suspension comprising particulate mattercan be filtered to either remove the particulate or to remove filtrateand increase the concentration of particulate matter in the remainingretentate in a manner in which the filter will not become clogged withparticulate matter.

It is another object of the present invention to provide an apparatusand method whereby maximum flexibility is assured so that therecirculation process can be performed at different rates from thefiltrate extraction process.

It is a further object of the present invention to provide an adjustablesystem wherein the recirculation and permeate extraction rate can beadjusted so that the maximum flux is achieved to keep the filter cleanand to adjust the rates to conform to the type of suspension beingfiltered.

Further novel features and other objects of the present invention willbecome apparent from the following detailed description, discussion andthe appended claims, taken in conjunction with the drawings.

DRAWING SUMMARY

Referring to the drawings for the purpose of illustration only and notlimitation, there is illustrated:

FIG. 1 is a schematic flow diagram of prior art filtration systems in avertical orientation, illustrating the direction of flow of both thesuspension and the filtrate.

FIG. 2 is a schematic flow diagram of prior art filtration systems in ahorizontal orientation, illustrating the direction of flow of both thesuspension and the filtrate.

FIG. 3 is a schematic flow diagram illustrating a key principle of thepresent invention, namely having the pump downstream from the filterunit, drawing the suspension through the filter unit with a negativepressure with respect to the retentate surface of the filter membrane.

FIG. 4 is a schematic flow diagram of the present inventionanti-clogging and dialysis device for filtration systems, illustratingthe off-positions of all three-way valves during the volume reductioncycle.

FIG. 5 is a schematic flow diagram of the present inventionanti-clogging and dialysis device for filtration systems, illustratingthe off-positions of all three-way valves during the wash-dialysiscycle.

FIG. 6 is an enlarged view of the filter used with the present inventionprinciples as positioned in FIGS. 4 and 5.

FIGS. 7 a. b. and c. are charts illustrating the dynamic nature of thepresent invention anti-clogging device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Although specific embodiments of the invention will now be describedwith reference to the drawings, it should be understood that suchembodiments are by way of example only and merely illustrative of but asmall number of the many possible specific embodiments which canrepresent applications of the principles of the invention. Variouschanges and modifications obvious to one skilled in the art to which theinvention pertains are deemed to be within the spirit, scope andcontemplation of the invention as further defined in the appendedclaims.

Referring to FIGS. 1 and 2, there is illustrated the direction of flowof both the suspension and the filtrate in conventional arrangements offilter units, in a vertical position, and in a horizontal position,respectively. In FIG. 1, the vertical flow filter arrangement 100illustrates the suspension 102 located in suspension holding tank 104.The pump 106 is located upstream of filter unit 108 and its filtermembrane 110. The pump 106 sucks suspension 102 out of suspensionholding tank 104 and pushes it with positive pressure toward filter unit108 and onto filter membrane 110. The filtrate 112 exits opening 114 offilter unit 108 while the retentate 116 exits the filter unit at itsbottom outlet opening 118 and flows back into the suspension holdingtank 104. In FIG. 2, the horizontal flow filter arrangement 120illustrates the suspension 122 located in suspension holding tank 124.The pump 126 is located upstream of filter unit 128 and its filtrationmembrane 130. The pump 126 sucks suspension 122 out of suspensionholding tank 124 and pushes it with positive pressure toward filter unit128 and onto filtration membrane 130. The filtrate 132 exits opening 134of filter unit 128 while the retentate 136 exits the filter unit at itsremote exit opening 138 and flows back into the suspension holding tank124. Examination of each of the filter units after filtration of asuspension will show that most clogging of the hollow fiber filtersoccurs at location A which is near the outlet of the filter unit, andmuch less at location B which is near the inlet of the filter unit. Thisis because there is a pressure gradient from the inlet of the filterunit to the outlet of the same unit. Therefore, the amount of cloggingis not uniform within the filter unit. What is common to both units isthe fact that the filter membranes (110 and 130) become clogged with theparticulate matter because the suspension is pushed with positivepressure onto the filter membrane (110 and 130 respectively) by the pumpwhich is located upstream of the filter.

A key principle of the present invention is illustrated in the schematicflow diagram of FIG. 3. The flow filter arrangement 140 illustrates thesuspension 142 located in suspension holding tank 144. The pump 146 islocated downstream of filter 148 and its filter membrane 150. The pump146 sucks suspension 142 out of suspension holding tank 144 and causesit to flow through filter unit 148 but pulling the retentate 156 awayfrom the filter membrane 150. The retentate 156 exits the filter unit atits remote opening 158 and is pulled away from the filter unit 148 bythe pump 146 which then causes the retentate to flow back into tank 144.FIG. 3 illustrates the importance of having the pump downstream from thefilter unit. With the jacket filled with suspensions of a differentcolor, it can be easily demonstrated that the flow of fluid from thejacket into the center of the hollow filter fibers is such thatparticulate matters from the suspension will be swept away from theretentate surface 150 of the filter 148.

Referring further to FIG. 3, a second pump 160 extracts filtrate orpermeate 172 through a side opening 162 in the filter unit wall. Thefiltrate 172 is collected in filtrate tank 170. The extraction offiltrate 172 creates a positive pressure on the filter membrane 150which would tend to clog it. However, a key principal of the presentinvention is that the pressure created from pump 146 is adjustedrelative to the pressure created from pump 160 so that there issufficient negative pressure to unclog the filter membrane 150 while atthe same time allowing a sufficient amount of filtrate 172 to passthrough the pores of the filter membrane 150. Overall, there is a netnegative pressure to keep the filter membrane 150 unclogged but not sogreat that all of the fluid is pulled by pump 146 without some of thefluid being pulled by pump 160.

The principles of the present invention are illustrated in a valvearrangement shown in FIGS. 4 and 5 which shown the adjustable negativepressure concept in one type of use. FIGS. 4 and 5 show the arrangementof various components including 3 pumps (two of which are principallyused), a filter unit, containers for the suspension to be concentratedor dialyzed, and rinsing or dialysis buffer. There are at least 5three-way valves, one three-way connection and one one-way valve in thedesign.

Of primary importance is the fact that the circulation pump (the onecirculating the bulk of the suspension between the container and thefilter unit) operates down-stream from the filter unit so that fluid isdrawn by negative pressure from the outlet of the filter unit. This isin contrast to most conventional filter units where the pump isup-stream from the filter unit, pushing the suspension toward the inletof the filter unit.

The present invention anti-clogging and dialysis device for filtrationsystems 10 comprises a suspension 12 inside a suspension holding tank14, a dialysis fluid 16 inside a dialysis fluid holding tank 18, and afiltrate or waste disposal tank 20. The device 10 further comprises afilter unit 22 having inlet opening 24, exit opening 26, first wallopening 28, second wall opening 30 and internal filter membrane 32. Thedevice 10 further comprises five valves, first valve 34, second valve36, third valve 38, fourth valve 40 and fifth valve 42. The device 10also comprises three pumps, first pump 44, second pump 46 and third pump48. A T-connection 50 joins first valve 34, fourth valve 40 and secondpump 46. A one way valve 52 is located between suspension tank 14 andfilter unit 22. An adjustable clamp 54 is located between second valve36 and first pump 44.

FIG. 4 illustrates the off-positions of all three-way valves (34, 36,38, 40 and 42) during the Volume Reduction (VR) cycle. The flow pathwayof the retentate 60 is separate from the flow pathway of filtrate 58.During this cycle, first pump 44 serves the purpose of a circulationpump, pumping the suspension 12 between the container and the filterunit. First pump 44 should ideally be rate-regulated. Second pump 46 isalso downstream from the filter unit 22 and serves to extract filtrate58 from the suspension 12. Second pump 46 ideally should berate-regulated so as to extract a constant amount of filtrate. Secondpump 46 should ideally be connected to the filtrate port 30 which isclosest to the outlet 26 of the filter unit 22. The filtrate extractionrate, i.e. amount of filtrate 58 extracted per unit time from the filterunit, is usually less than the re-circulation rate, i.e. amount ofretentate 60 leaving the filter unit 22 per unit time. In thisembodiment, third pump 48 is turned off and the flow paths are asfollows. First pump 44 sucks suspension 12 out of the suspension holdingtank 14 and causes it to go through filter unit 22, entering throughentrance 24 and being pulled inwardly away from filter membrane 32 (asillustrated in FIG. 3) and out exit opening 26, through first valve 34,second valve 36, through first pump 44, through third valve 38 and backinto suspension holding tank 14. Second pump 46 sucks the filtrate 58out lower portion or second sidewall opening 30, through fourth valve40, through T-section 50, through second pump 46, through fifth valve 42and into waste disposal tank 20.

The design of this device is such that the recirculation rate ispreferably greater than the filtrate extraction rate to insure thatparticulate matters do not clog the filter pores. Depending on thenature of the suspension being filtered, as discussed below, thefiltrate extraction force may sometimes be greater than therecirculation force. The reason for such an arrangement is to makecertain that the overall negative pressure pulling particulate mattersaway from filter pores 32 is always greater than the overall positivepressures pressing the particulate matters against the pores to clogthem.

The ratio of the recirculation rate to the filtrate extraction rate,however, is affected by a number of factors. For example, a filter unitplaced in a vertical position will have the gravitational forcefacilitating the flow of retentate in a vertically down direction (suchas in FIG. 4), naturally unclogging particles from the retention surfaceof the filter membrane. Therefore, even for a slow recirculation rate ofabout 10 cc/min, there is enough negative force unclogging the filter,that the filter membrane will not be clogged when a filtrationextraction rate of 8 cc/min is applied. However, when the filter unit isplaced in a horizontal position gravitational force tends to work insuch a way that particulate matter will settle down inside the hollowfilter fibers, creating a condition favorable to clogging. Therefore, agreater negative pressure is needed to unclog the filter membrane, e.g.by using at least a recirculation rate of 20 cc/min when an 8 cc/minfiltration extraction rate is desired. Other factors will includeviscosity of the fluid phase and how easy it is for the fluid to passthrough the filter pores. Lower viscosity of the fluid and larger poresize will facilitate exit of the fluid with minimal positive pressure,thus allowing a higher filtrate extraction rate or flux. The pore sizein the filter membrane compared to the particulate size will also affectthe optimal filtrate extraction rate vs the recirculation rate. If theparticulate matters are much larger than the pore size, which means theydo not clog easily anyway, a higher filtrate extraction rate again canbe tolerated without proportionally increasing recirculation rate toincrease the negative unclogging pressure. This design is truly ananti-clogging device because after the overall adjustment of the system,negative pressures on particulate matters are designed to be moving themaway from any part of the filter rententate surfaces while positivepressures move the fluid phase of the suspension through the filterpores as permeate.

Even during temporary disequilibrium of pressures at a given locationalong the filter fibers, the device is designed to unclog obstructedfilter pores. The causes of pressure disequilibrium are many, butprimarily because there are pressure gradients along different parts ofthe filter. Consider the simple case of any single filter pore, X,located at any part of the hollow filter fiber inside the filter unit.Temporary disequilibrium of pressure may cause the positive pressure atX to exceed that of the negative pressure created by the pulling forceof the recirculation pump. The resultant net positive pressure willcause pore X to be temporarily clogged. Once pore X is clogged, thepositive pressure created by the filtrate pump will decrease at X tozero, because flow rate there becomes zero. (Flow rate is equal topressure divided by resistance. When X is obstructed, resistance at Xbecomes infinitely large and so the pressure in the direction of theflow of fluid becomes zero). Since the negative pressure at X created bythe pulling force of the recirculation pump remains unchanged, adecrease of positive pressure at X to zero will allow an increasinglylarger net negative pressure to unclog again the particulate matterstemporarily obstructing X.

FIG. 5 illustrates the off-positions of all three-way valves during theWash-Dialysis (W) cycle. During this cycle, second pump 46 serves as thecirculation pump. First pump 44, which has a faster rate, will push arelatively large amount of dialysis buffer 16 across the filter unit tobathe the filter fibers and by diffusion, to change the composition ofthe soluble phase of the suspension. If it is not desirable to changethe composition of the soluble phase of this suspension, a fresh mediumsimilar in composition to the permeate can be used. The purpose of suchan infusion will be mainly to augment the negative pressures atlocations of the filter membrane near to the inlet 24 of the filterunit. Third pump 48 will be set at a rate at least equal to that offirst pump 44, for the purpose of extracting the used dialysis fluid outof the filter unit at the same rate so that excess dialysis fluid willnot exit the filter outlet 26 to flood the retentate and thus lower theconcentration of the washed particulate matters. If it is desirable toperform both volume reduction as well as dialysis function at the sametime, third pump 48 can be set at a rate slightly larger than that offirst pump 44. When the rates are thus set, the volume of dialysis fluidextracted from the filter unit per unit time will be greater than theamount delivered. The difference must have come from the soluble phaseof the suspension, thus reducing the volume of the recirculatedsuspension. Again, the difference between the rate of first pump 44(dialysis fluid delivery) and third pump 48 (filtrate extraction rate),defined as volume reduction rate, is usually less than the retentaterecirculation rate. Otherwise, the positive pressure generated by thenet extraction of the soluble phase may be excessive compared to thenegative pressure generated by the recirculation pump 46, leading toclogging of the filter. However, as discussed above, certain suspensionswill allow volume reduction rate to be greater than the retentaterecirculation rate. Volume reduction rate in the absence of dialysisfluid delivery is identical to permeate or filtrate extraction rate.

Of importance is the presence of the one-way valve 52 so placed thatlarge amounts of dialysis buffer 16 will not back-flow into thesuspension container and so increase the total volume (thus reducing theconcentration of suspended particulate matters).

If a filter 22 has more than one filtrate port, the one farthest awayfrom the outlet of the filter unit (first opening 28) should be used forinfusion of dialysis fluid. In contrast, the filtrate port closest tothe filter outlet (second opening 30) should be used for filtrate exit.If the wrong connections are made, such as using opening 30 for infusionand opening 28 for exit of dialysis fluid, dialysis fluid willpreferentially exit from the filter outlet 26 instead of opening 28.This is because the filter pores near the filter unit outlet port willexperience the greatest negative pressure from the recirculating pump.In addition, the direction of flow of the dialysis fluid, when infusedat second opening 30 under pressure, will also create a negativepressure (with respect to the retentate surface 32 of the filter 22,which in the hollow fibers is the surface facing interior) at the poresnear the filter outlet. Dialysis fluid will therefore enter into theinterior of the hollow filter filters. Thus the combined greaterincreased negative pressure will draw the dialysis fluid (in addition tothe suspension) out of the filter unit. When this happens, the retentate60 will be diluted.

Another disadvantage of the wrong connection is that the pores near thefilter inlet 24 will be clogged. The reason for the clogging is that thenegative pressure generated by the recirculating pump is weakest at thepores near the filter inlet. In addition, part of the dialysis fluid,which had entered into the interior of the hollow fibers from pressureat opening 30, is now drawn toward opening 28 by the pulling of thirdpump 48. The direction of flow of fluid, from the interior of the fibersoutwards, is positive with respect to the retention surface there.Therefore, for the pores near the filter inlet 24, the net pressure isgreatly positive and thus tends to push the particulate matters onto thepores to clog them.

However, if the connections are properly arranged, such as in FIG. 5,with infusion of dialysis fluid through opening 28, and fluid extractionat opening 30, the design is at optimal condition for washing, or thecombined function of volume reduction and washing. The direction of flowof dialysis fluid will be con-current with the flow of the suspensioninside the hollow fibers. The pressure generated by second pump 46 issuch that it creates a large negative pressure on the filter pores nearthe filter outlet 26 which will counter the pulling force of third pump48 which generates a positive pressure (again with respect to theretention surface 32) on the filter membrane. The negative pressuregenerated by pump 46 on the filter pores near the filter inlet 24 is notas great as that near the outlet 26, but the negative pressure isaugmented by the negative pressure generated (again with respect toretentate surface 32) by first pump 44. Again, to insure that thenegative pressure is adequate for anti-clogging as compared to thepositive pressure near outlet 26, the difference between first pump 44and third pump 48 (filtrate extraction rate) or the volume reductionrate preferably should be remain less than that of the retentaterecirculating pump.

In practice, the outer jacket of the filter unit should be completelyprimed with a suitable fluid (e.g. dialysis fluid) before all the pumpsare turned on; especially the filtrate pump to extract the filtrate.This will insure that pressure is evenly distributed within the filterunit and that filtrate is extracted from the entire columns of filterfibers and not just from the portions of filter fibers near the outletof the filter unit. The mechanism for priming the filter unit depends onthe design of the filter unit. Some units are marketed with primaryfluid inside the jacket (e.g. Curesis Plasma Separator). For those thatcome without a suitable priming fluid, pump 44 can deliver the dialysisfluid to the jacket until it is filled up from the bottom to the top.During priming, pump 46, 48 will be turned off and the one way valve 52turned in a reverse direction to allow air to escape the filter unituntil all tubings are completely primed. Then valve 52 will be turnedback to its normal direction and the rest of the pumps set at theirdesirable pump rates, will be turned on for either the VR or W, or VRand W cycle.

Although presented conceptually here as separate "pump systems" such asfirst pump 44, second pump 46 and third pump 48, in practice a singlepump with two heads can be used to combine the function of the firstpump 44 and third pump 48 presented here; with a separate second pump 46to be used nearby. An even more sophisticated pump with different gearratios can also be employed to do the work of all three pumps, pullingthe retentate 60 and the filtrate 58 at the desired ratio ofrecirculation to filtrate extraction flow rates.

In practice, an adjustable clamp 54 can be placed between valve 36 andvalve 38 as illustrated in FIG. 5, so that slight compression of thedialysis fluid infusion tubing will decrease the rate of delivery toopening 28 as compared to extraction of fluid from opening 30. Such anarrangement will permit a single pump with a dual-head to have one pumpspeed and yet deliver less fluid to opening 28 than extraction of fluidfrom opening 30, to achieve simultaneous volume reduction andwash-dialysis (VR+W) functions.

The present invention concept will now be described in detail. FIG. 6 isa filter unit comparable to the filter units in FIGS. 4 and 5, but shownin much greater detail. The essential features of the anti-cloggingprinciple as applied to hollow fibers ultrafiltration systems will nowbe described. Pump 1 (comparable to pump 44 in FIGS. 4 and 5) is thepump which is used to pull the suspension 12 through the filtercartridge 22 and then recirculate the retentate 60 back to thesuspension container 14. The direction of flow of the suspension 12within the cartridge is essentially vertically down. The membranepressure created is negative inside the hollow fibers and tends to moveparticles or other resistance materials away from the retentate surfaceof the filters 32. Pump 2, comparable to pump 46 in FIGS. 4 and 5 isused to pull the permeate through the hollow fiber filters 32. Eventhough pump 2 generates a "pulling" force, it tends to impactparticulate matters onto the retentate surface (in this case theinterior surface) of the filter 32 and therefore exerts a competingpositive membrane pressure. In a cartridge with two permeate exit ports,the lower one 30 should be connected to pump 2. Such an arrangement willallow the maximal positive membrane pressure experienced by any filterfiber (such as that created by pump 2 at the lower portions of thecartridge) to be overcome by an appropriately matched negative membranepressure at the same site. In this arrangement, the negative pressurecreated by pump 1 is greatest at the lower portions of the cartridge.The key to success of the anti-clogging system is to match the greatestnegative pressures to sites where the greatest positive pressures areexpected in the system, to constantly keep the filters unobstructed.

Pump 3 is optional, but it is useful for the dialysis of the suspension12, or the addition of other soluble material, if desirable, to thesuspension at the same time the permeate 58 is extracted from the unit.The rate of infusion by pump 3 can be greater than, equal to or lessthan the rate at which the permeate leaves the permeate exit port,depending on the purpose of the experiment. Even if there is no need tochange the composition of the soluble fraction of the suspension, pump 3can be used to augment the anti-clogging function of pump 1 by infusionof a solution similar in composition to that of the permeate. Pump 3should be connected to the upper side-port 28 so that the flow ofinfusate is concurrent (and not countercurrent) to the flow of thesuspension inside the hollow fibers. The negative membrane pressuregenerated by pump 1 is the weakest at the upper portions of the filterfibers. Under optimal conditions, such negative membrane pressure maystill be able to overcome the positive membrane pressure generated bypump 2 that tends to promote clogging or gel-layer formation. Undernon-ideal conditions where the positive membrane pressure created bypump 2 is excessive compared to the negative membrane pressure generatedby pump 1, such as can occur in the upper portions of the filter fibers,leading to excessive clogging there, pump 3 can be used to augment thenegative membrane pressures. Even though pump 3 pushes fluids into thecartridge, it actually creates a negative membrane pressure at the upperportions of the filter fibers because the fluids tend to move fromoutside the fibers inwards and thus tend to unclog resistance materialsfrom the "pores" situated there. If pump 3 is not in use, the upperside-port 28 should be closed and the negative pressure of pump 1 alonemust be great enough to provide anti-clogging negative pressures for theentire length of the fiber from bottom to top.

Several flow rates are characteristic of the anti-clogging pressurearrangements. Volume reduction rate is equal to permeate outflow rateminus infusate inflow rate. Therefore volume reduction rate is identicalto permeate outflow rate if infusate inflow rate is zero. Depending uponthe amount of solid material in the suspension, retentate recirculationrate is often greater than volume reduction rate to prevent gel-layerformation. The ratio of retentate recirculation rate to volume reductionrate is a reflection of the overall negative membrane pressure over theoverall positive membrane pressure along the length of the fibers. Forviscous material, we expect that this ratio is high.

While it is more practical to describe the present disclosure with fluidflow rates, the present invention can also be illustrated by diagramsusing pressure gradients. While not adhering to any particularscientific theories, the mechanism with which the present inventionworks can be described as follows: FIG. 7 explains the dynamic nature ofsuch an anti-clogging device. FIG. 7-A describes the initial conditionas pump 1 is first turned on and then pump 2 is turned on. For the sakeof illustration, the interior pressure at the bottom of a filter fiberis depicted as -30 psi. Since it tends to pull particles away from theretention surface of the fiber, it creates a negative membrane pressurethere. Let us start with a pump 2 setting such that pump 2 pulls at anexcessively high pressure (or flow rate) e.g. -60 psi. As defined above,pump 2 creates a positive membrane pressure which tends to promotegel-layer formation or impaction of particulate matters onto theretention surface of the filter. Pump 3 is arranged so that it will pushfluids into the cartridge by positive (e.g. +5) psi. In this condition,the fluids will flow from outside the fibers into the interior of thefibers at the upper portion of the fibers such as above point Y(approximately the upper 2/5 of the length of the filter unit), but thepermeate will flow outwards from the interior of the fibers at the lowerportions of the fibers, such as below point Y.

As more permeate is extracted and the concentration of solid materialsin the suspension builds up, the gel-layer or clogging effect willbecome more apparent, particularly at the lower portion of the fiberswhere the positive membrane pressure created by pump 2 is the strongest.FIG. 7-B depicts a drastic condition that often occurs during thefiltration process: the suspension actually contains particles which caneffectively and completely occlude the "pores" of the filter. We willexamine the effect on pore X (of FIG. 6) which is a single filter porethat can be situated on any location of a fiber. Fluids are flowing fromthe interior of pore X outwards until it is suddenly obstructed. Sinceresistance is complete with a perfect occlusion, flow outwards at pore Xdrops to zero. Pump 2 will continue to draw permeate but now thepermeate comes from some other still open pores. At this time, thepositive membrane pressure at pore X will effectively become zero. (Flowis equal to pressure gradient divided by resistance). However, thenegative membrane pressure maintained by pump 1 is still in effect.Therefore, the unopposed negative membrane pressure created by pump 1unplugs the pore X by promoting flow of fluid from the jacket into theinterior of the fiber at location X. FIG. 7-C describes the inward andoutward pressure relationships along the length of the fiber as "steadystate" within the entire cartridge is reached. Actually for a givenlocation or a certain "pore" on the fiber, the "steady state" is aconstant fluctuation between outward flow and inward flow of the fluid.Every time the solid material occludes the membrane or forms a gel-layerwhere flow from the interior of the fiber outwards is impeded, thenegative membrane pressure will take over to disperse that layer.Meanwhile, at a different site where the gel-layer has only begun tobuild, positive membrane pressure is fully at work to maximally extractthe permeate. Thus the negative anti-clogging pressures are, by thisdesign, most effective where obstruction to flux is the greatest whilethe positive membrane pressure works to extract permeate at locationswhere obstruction to flux is the least. This arrangement allows themembrane to be cleansed on a continuous basis and will not promotecomplete, irreversible occlusion of the filter or build-up ofgel-layers.

Therefore, the present invention can be defined as an apparatus forfiltering a suspension and increasing the concentration of retainablematter in the retentate, comprising: (a) a suspension holding means; (b)a suspension including particulate matter retained within saidsuspension holding means; (c) means for capturing permeate; (d) a filtermeans further comprising a housing having an inlet port, an exit port,at least one sidewall opening, and a filter membrane within the housing;(e) a first pump means; (f) a second pump means; (g) an interchangeablevalve means; (h) means for interconnecting said first pump means, saidsecond pump means, and said interchangeable valve means to said filtermeans, to said suspension holding means, and to said means for capturingpermeate; (i) said first pump means and said second pump means eachlocated downstream of said filter means; (j) said interchangeable valvemeans having a setting wherein said first pump means causes a portion ofsaid suspension to be drawn out of said suspension holding means and runthrough said filter means without passing through said filter membraneand recirculated back into the suspension holding means while saidsecond pump means causes a portion of said suspension to pass throughsaid filter membrane to form permeate from the fluid phase of thesuspension which has passed through said filter membrane and be drawnthrough said at least one sidewall opening in the filter housing anddirected to said means for capturing permeate; and (k) said first pumpmeans and said second pump means adjusted relative to each other suchthat the second pump means exerts a positive pressure on the suspensionrelative to the filter membrane to cause a portion of the fluid phase ofthe suspension to flow through the filter membrane and the first pumpmeans exerts a negative pressure on the suspension relative to thefilter membrane to cause a portion of the suspension to flow through thefilter means without passing through the filter membrane, such that asthe portion of suspension passing through the filter membrane clogs alocation of the filter membrane with particulate matter, the flow of thesuspension which does not pass through the filter membrane creates anegative pressure relative to the filter membrane to pull theparticulate matter away from the filter membrane and unclogs it.

Depending on the suspension, in some applications the negative pressureexerted on the suspension from said first pump means causes a retentaterecirculation rate always greater than the volume reduction rate createdby the positive pressure exerted on the suspension from said second pumpmeans. In other applications, the positive pressure exerted on thesuspension from said second pump means causes a volume reduction ratealways greater than the retentate recirculation rate created by thenegative pressure exerted on the suspension from said first pump means.In still other applications, the flow rates created by said first pumpmeans and said second pump means are varied such that during part of thefiltration process the retentate recirculation rate created by negativepressure exerted on the suspension from said first pump means is greaterthan the volume reduction rate caused by the positive pressure exertedon the suspension from said second pump means and during part of thefiltration process the positive pressure exerted on the suspension fromsaid second pump means results in a volume reduction rate greater thanthe retentate recirculation rate created by the negative pressureexerted on the suspension from said first pump means.

Defined more broadly, the present invention is an apparatus forfiltering a suspension including particulates and increasing theretainable matter in the retentate, comprising: (a) means for retainingsaid suspension; (b) means for filtering said suspension including afilter membrane; (c) means for drawing suspension out of the means forretaining said suspension and causing the suspension to pass throughsaid means for filtering said suspension, wherein the means exerts anegative pressure on the suspension relative to the filter membrane andcauses the suspension to be drawn away from the filter membrane and passthrough the filter means for recirculation back to the means forretaining said suspension; (d) means for exerting a positive pressure onthe suspension relative to the filter membrane as the suspension ispassed through the filter means to thereby cause a portion of the fluidphase of the suspension to pass through the filter membrane and create apermeate; and (e) adjusting the means for exerting a negative pressureon the suspension and the means for exerting a positive pressure on thesuspension relative to each other such that as the portion passingthrough the filter membrane clogs a location of the filter membrane withparticulate matter, the flow of the suspension which does not passthrough the filter membrane creates a negative pressure relative to thefilter membrane to pull the particulate matter away from the filtermembrane and unclogs it.

The present invention is also defined as a method for filtering asuspension including particulate matter and increasing the concentrationof retainable matter in the retentate, comprising: (a) causing saidsuspension to be drawn through a filter means whereby a portion of thesuspension is recirculated through the filter means without passingthrough the filter membrane; (b) causing said suspension to be drawnthrough a filter means whereby a portion of the fluid phase of thesuspension is passed through the filter membrane to create permeate; and(c) adjusting the flow of suspension which does not pass through thefilter membrane and the portion of suspension which does pass throughthe filter membrane relative to each other such that as the portionpassing through the filter membrane clogs a location of the filtermembrane with particulate matter, the flow of the suspension which doesnot pass through the filter membrane creates a negative pressurerelative to the filter membrane to pull the particulate matter away fromthe filter membrane and unclogs it.

The present invention is also defined as a method for dialysing asuspension including particular matter with or without concomitantreduction of the volume in which the particular matter is suspended.

The method of the present invention further involves a matching of thepressures to maximize the anti-clogging effect, wherein the location onthe filter means having the greatest potential to clog due to positivepressure at the location is matched with the greatest negative pressureto most effectively unclog it to maintain a continuous process ofanti-clogging filtration.

The method also includes a supplemental dialysis process for changingthe composition of the fluid phase of the suspension. By infusing afresh fluid different in composition to the permeate at a site near theinlet of the filtration means and causing the direction of flow of thefresh fluid to be concurrent to the direction of flow of the suspension,the composition of the fluid phase of the suspension can be changed,with and without concomitant reduction in volume of the suspension.

The method also includes a supplemental process for augmenting thenegative pressure at locations of the filter means which are subjectedto excessive positive pressure. Such excessive positive pressure isusually due to the length of the filter fibers in the filter means whichdoe snot allow adequate negative pressure at those locations to promoteanti-clogging function. By infusing a fresh fluid similar in compositionto the permeate at a site near the inlet of the filtration means andcausing the direction of flow of the fresh fluid to be concurrent to thedirection of flow of the suspension, additional negative pressure isapplied to the locations with otherwise excessive positive pressures, toproduce anti-clogging effects in those locations.

The method also includes dialysing the suspension with concomitantreduction in volume of the suspension, further comprising: (a) infusinga fresh fluid different in composition from the fluid phase of thesuspension at a site near the inlet of the filtration means; (b) causingthe rate of infusion of said fresh fluid to be less than the rate ofpermeate extraction from the suspension; and (c) causing the directionof flow of said fresh fluid to be concurrent to the direction of flow ofthe suspension.

The method also includes dialysing the suspension without concomitantreduction in volume of the suspension, further comprising: (a) infusinga fresh fluid different in composition from the fluid phase of thesuspension at a site near the inlet of the filtration means; (b) causingthe rate of infusion of said fresh fluid to be equal to the rate ofpermeate extraction from the suspension; and (c) causing the directionof flow of said fresh fluid to be concurrent to the direction of flow ofthe suspension.

Of course the present invention is not intended to be restricted to anyparticular form or arrangement, or any specific embodiment disclosedherein, or any specific use, since the same may be modified in variousparticulars or relations without departing from the spirit or scope ofthe claimed invention hereinabove shown and described of which theapparatus shown is intended only for illustration and for disclosure ofan operative embodiment and not to show all of the various forms ofmodification in which the invention might be embodied or operated.

The invention has been described in considerable detail in order tocomply with the patent laws by providing full public disclosure of atleast one of its forms. However, such detailed description is notintended in any way to limit the broad features or principles of theinvention, or the scope of patent monopoly to be granted.

What is claimed is:
 1. An apparatus for filtering a suspension andincreasing the concentration of retainable matter in the retentate,comprising:a. a suspension holding means; b. a suspension includingparticulate matter retained within said suspension holding means; c.means for capturing permeate; d. a filter means further comprising ahousing having an inlet port, an exit port, at least one sidewallopening, and a filter membrane within the housing; e. a first pumpmeans; f. a second pump means; g. an interchangeable valve means; h.means for interconnecting said first pump means, said second pump means,and said interchangeable valve means to said filter means, to saidsuspension holding means, and to said means for capturing permeate; i.said first pump means and said second pump means each located downstreamof said filter means; j. means for enabling said interchangeable valvemeans to be set wherein said first pump means causes a portion of saidsuspension to be drawn out of said suspension holding means and runthrough said filter means without passing through said filter membraneand recirculated back into the suspension holding means while saidsecond pump means causes a portion of said suspension to pass throughsaid filter membrane to form permeate from the fluid phase of thesuspension which has passed through said filter membrane and be drawnthrough said at least one sidewall opening in the filter housing anddirected to said means for capturing permeate; and k. means for enablingsaid first pump means and said second pump means to be adjusted relativeto each other such that the second pump means exerts a positive pressureon the suspension relative to the filter membrane to cause a portion ofthe suspension to flow through the filter membrane and the first pumpmeans exerts a negative pressure on the suspension relative to thefilter membrane to cause a portion of the suspension to flow through thefilter without passing through the filter membrane, such that as theportion of suspension passing through the filter membrane clogs alocation of the filter membrane with particulate matter, the flow of thesuspension which does not pass through the filter membrane creates anegative pressure relative to the filter membrane to pull theparticulate matter away from the filter membrane and unclog it.
 2. Anapparatus in accordance with claim 1 wherein the flow rate of thesuspension not passing through the filter membrane created by thenegative pressure exerted on the suspension from said first pump meansis always greater than the volume reduction rate of the suspensioncaused by the positive pressure exerted on the suspension from saidsecond pump means.
 3. An apparatus in accordance with claim 1 whereinthe volume reduction rate of the suspension caused by the positivepressure exerted on the suspension from said second pump means is alwaysgreater than the flow rate of the suspension not passing through thefilter membrane created by the negative pressure exerted on thesuspension from said first pump means.
 4. An apparatus in accordancewith claim 1 wherein the flow rate of the suspension not passing throughthe filter membrane created by the negative pressure exerted by saidfirst pump means and the volume reduction rate caused by said secondpump means are varied such that during part of the filtration processthe flow rate of the suspension not passing through the filter membraneis greater than the volume reduction rate of the suspension and duringpart of the filtration process the volume reduction rate of thesuspension is greater than the flow rate of the suspension which doesnot pass through the filter membrane.
 5. An apparatus for filtering asuspension including particulates and increasing the retainable matterin the retentate, comprising:a. means for retaining said suspension; b.means for filtering said suspension including a filter membrane; c.means for drawing suspension out of the means for retaining saidsuspension and causing the suspension to pass through said means forfiltering said suspension, wherein the means exerts a negative pressureon the suspension relative to the filter membrane and causes thesuspension to be drawn away from the filter membrane and pass throughthe filter means for recirculation back to the means for retaining saidsuspension; d. means for exerting a positive pressure on the suspensionrelative to the filter membrane as the suspension is passed through thefilter means to thereby cause a portion of the suspension to passthrough the filter membrane and create a permeate; and e. means foradjusting the means for exerting a negative pressure on the suspensionand the means for exerting a positive pressure on the suspensionrelative to each other such that as the portion passing through thefilter membrane clogs a location of the filter membrane with particulatematter, the flow of the suspension which does not pass through thefilter member creates a negative pressure relative to the filtermembrane to pull the particulate matter away from the filter membraneand unclogs it.
 6. An apparatus in accordance with claim 5 wherein saidmeans for recirculating said suspension is located downstream from saidfilter means.
 7. An apparatus in accordance with claim 5 wherein theflow rate of the suspension created by the negative pressure exerted onthe suspension from said means for exerting a negative pressure isalways greater than the volume reduction rate created by the positivepressure exerted on the suspension from said means for exerting apositive pressure.
 8. An apparatus in accordance with claim 5 whereinthe volume reduction rate created by the positive pressure exerted onthe suspension from said means for exerting a positive pressure isalways greater than the flow rate of the suspension created by thenegative pressure exerted on the suspension from said means for exertinga negative pressure.
 9. An apparatus in accordance with claim 5 whereinthe flow rate of the suspension created by said means for exerting anegative pressure and the volume reduction rate created by said meansfor exerting a positive pressure are varied such that during part of thefiltration process the flow rate of the suspension created by thenegative pressure exerted on the suspension from said means for exertinga negative pressure is greater than the volume reduction rate created bythe positive pressure exerted on the suspension from said means forexerting a positive pressure and during part of the filtration processthe volume reduction rate created by the positive pressure exerted onthe suspension from said means for exerting a positive pressure isgreater than the flow rate of the suspension created by the negativepressure exerted on the suspension from said means for exerting anegative pressure.